1. Field of the Invention
The present invention relates to a shielding technique of neutrons in a reactor core, and particularly, to a neutron shield for absorbing neutrons leaking from a nuclear reactor core at an outside of the core.
2. Description of the Related Art
Conventionally, there are proposed technologies utilizing powder of a neutron absorbing material such as B4C for the use of a control rod (for example, Patent Document 1: Japanese Patent Laid-Open Publication No. 10-39072). However, as a neutron shield structure disposed outside a nuclear reactor core for absorbing neutrons leaking from the core, pellets formed by baking a neutron absorbing material such as B4C are used. However, it has not known up to the present that a powdered neutron absorbing material has been used for the neutron shield.
A neutron shield is disposed outside the core of a nuclear reactor in a close packed manner so as to cover an outer periphery of the nuclear reactor core and performs a function to protect a nuclear reactor vessel from fast neutrons having high energy. However, unlike the situation for control rods, such a neutron shield is regarded as an internal structure of the reactor and is neither repaired nor replaced for a predetermined period of time, thus requiring a long-term soundness.
However, for such conventional technology, when a pellet-type material is used as the neutron absorbing material, thermal expansion and swelling of the pellets due to neutron irradiation causes a structural degradation of the neutron shield, which makes it difficult to maintain the soundness of the neutron shield until the time of repair or replacement. For example, there is a risk that the pellets are crushed due to thermal expansion and swelling, and hence, damages of the neutron shield or other internal structures of the reactor are caused. In addition, it is not negligible that when the neutron absorbing material is formed into pellets, a baking process of the neutron absorbing material is required, which leads to a high manufacturing cost.